1. Field
Circuit fabrication, including a method for forming a heterojunction bipolar transistor.
2. Description of the Related Art
Bipolar transistors, more specifically heterojunction bipolar transistors (HBTs) are used in devices requiring high frequency operation such as wireless and networking devices. HBTs are used in these devices because of their high cut off frequencies greater than 150 gigahertz (Ghz) even though they consume more power than equivalent metal oxide semiconductor (MOS) based technologies.
HBTs typically consist of an emitter region, base region and collector region. The emitter region generally has a larger band gap than the base region to achieve high frequency performance. The speed at which the HBT can switch is referred to as the cutoff frequency, ft. The cutoff frequency of a given HBT is generally related to the width of its base region. The narrower the base region of a HBT, the shorter the base transit time and higher the cutoff frequency, ft.
HBTs are typically formed on a silicon substrate and the base region is typically formed in silicon germanium (SiGe) films. For example, as illustrated in FIG. 1, a generic HBT 10 includes a collector layer 12 formed on the surface of a silicon substrate 11. The substrate 11 may include an epitaxial layer 13 and device isolation regions (Shallow Trench Isolation, STI) 14 which is typically comprised of silicon oxide. A thick layer of SiGe film containing boron is formed on the substrate 11 and over the entire surface area of the epitaxial layer 13. The SiGe film is used to form the SiGe base region 17 of the HBT 10. An emitter 18 is formed over an area of the SiGe base region 17. The emitter 18 comprises of a silicon film. Contacts are also provided for the emitter 18, the collector layer 12, and the SiGe base region 17 as shown in FIG. 1 (e.g., contacts 20, 22, and 24).
Current methods for depositing the SiGe film to form the base region have several disadvantages. As can be seen from FIG. 1, the SiGe film is typically deposited over different surfaces, for example, over the epitaxial layer 13 and the isolation regions 14 which is usually a silicon oxide layer. It is thus common that the SiGe film is deposited over different surfaces that include a silicon surface and an oxide surface. There is a significant difference in nucleation time, residence time, sticking coefficient, or deposition rate of the SiGe film over the silicon and the oxide surfaces. Depositing the SiGe film over the different surfaces using the current method leads to non-uniform and discontinuous film. One reason for that is that the SiGe film is deposited more selectively over silicon surface than on oxide surface.
Discontinuous deposition of the SiGe film results in discontinuous base contact, which leads to an increase in contact resistance and/or extrinsic base resistance. Increase resistance in turn results in a degradation of Fmax (Oscillation frequency).